The present invention relates to micromechanical systems (MEMS) devices, particularly though not exclusively to such systems formed on a nanometer scale.
Commonly, MEMS devices are integrated within a monolithic integrated circuit, but the additional processing required for a required function is very product specific and often cumbersome. Many of the devices depend on vertical movement of suspended beams, which necessitates removal of the material under the beam, which is a problem; see for example EP-A-0932171.
Devices using cantilever beams, which deflect under an applied force, are well known. Cantilevers, which deflect under the weight of molecules absorbed on the surface of the beams are disclosed in A Boisen, J Thaysen, H Jensenius, O Hansen, Ultra Microscopy, 82, 11 (2000). A large array of interdigital cantilevers which move under the weight of absorbed molecules is disclosed in T Thundat, E Finot, Z Hu and R H Ritchie, Applied Physics Letters 77 4061 (2000). In this arrangement deflection of the elements of the array changes the diffraction pattern of an incident beam. xe2x80x9cTranslating Biomolecular Recognition into Nanomechanicsxe2x80x9d Fritz et al, Science, Vol. 288, 14 Apr. 2000, pp 316-319 discloses cantilevers which selectively bend as a result of surface stress changes caused by specific transduction of DNA hybridisation and receptor ligand binding to provide a true molecular recognition signal.
In another application, a reflective polarizer consisting of two layers of 190 nm period metal gratings is fabricated using nanoimprint lithography: see xe2x80x9cReflective Polarizer Based on a Stacked Double-Layer Sub-Wavelength Metal Grating Structure Fabricated Using Nanoimprint Lithographyxe2x80x9d, Z Hu, P Deshpandy, W Wu, J Wang, S Y Chou, Applied Physics Letters, volume 77, no. 7, 14 Aug. 2000.
It is an object of the invention to provide an improved MEMS device for fabrication in a comparatively simple and inexpensive manner, at nanometer dimensions.
The concept of the present invention is to provide an array of nanometric dimensions consisting of two or more longitudinal elements, positioned side by side, wherein the beams are of such nanometric dimensions that the elements can be moved or deformed towards or away from one another by means of a voltage differential applied between the elements, whereby to produce a desired optical, electronic or mechanical effect.
For the purposes of this specification, the term xe2x80x9clongitudinal elementxe2x80x9d includes fingers, arms, legs, longitudinal elements, or lines.
The invention may in some forms include two dimensional elements such as plates.
We have realised that at nanometer scale dimensions structures previously treated as rigid become flexible. Furthermore this flexibility can be engineered since it is a direct consequence of its material and dimensions. As preferred the dimensions of the elements are typically of a width, between 10 and of the order of 100 nanometers and of a length of the order of micrometers. The elements are spaced apart a distance between 10 and of the order of 100 nanometers. Since the electrostatic force between two elements is inversely proportional to the square of the distance, a very considerable force will be developed with a low voltage of the order of 1 or 2 volts or 5 volts, which is sufficient to deflect the elements towards or away from one another.
Among the various possibilities for mounting the elements, it in not preferred to have them fixed only at one end and for them to be freely moveable under the influence of an electric field along their length towards and away from one another. A disadvantage of this in terms of processing in that it is necessary to etch or otherwise remove material from beneath the elements so that they are spaced from the substrate to enable movement. In a preferred form of the invention, the elements are fixed to an underlying substrate, preferably along their whole length, or at the least, at two spaced apart points along their length (preferably one point is at one end), so that when an electrostatic field is applied across the elements by means of a voltage differential, the upper and distal parts of the elements remote from the fixing bend relative to one another, whereas lower parts of the elements and parts close to the fixings of the elements, remain stationary. The advantage of such a system is that it is easy to manufacture by means of known manufacturing techniques, but yet provides sufficient movement for desired functions.
For the purpose of this specification, xe2x80x9cupperxe2x80x9d and xe2x80x9clowerxe2x80x9d are to be understood in a relative sense, and do not imply any orientation of the device relative to its environment.
As preferred, the bulk of the element may be formed from either an insulating material, and then an upper conductive layer is applied on the upper surface, or alternatively the element may be formed completely of conductive material and then there is no need to apply a top conductive layer. These configurations permit a voltage differential to be developed across the elements.
The insulating material of the element may be, where the element is formed by a nanolithography method such as nanoimprint lithography (NIL), a resist such as PMMA, which is used in the NIL process.
Alternatively where the elements are formed by a CMOS metalization process, or by the nano xerography process as disclosed and claimed in our international application WO 01/84238, the elements may be formed of any desired material which is suitable for use in the process (such as a metal or semiconductor or insulating material).
The device according to the invention may be used in a variety of applications:-
a variable capacitorxe2x80x94filter circuit. By applying a bias voltage between two electrode elements, the distance between them is changed and therefore the capacitance. An AC signal passed through the capacitance thus experiences a capacitance dependent on the bias voltage.
GHzxe2x80x94resonator/RF applications. Because of the very small size of the elements they would show a mechanical resonance frequency in the GHz range. This frequency can be tuned by applying a potential.
Biosensorxe2x80x94few or single molecule detection. The resonance properties mentioned above can be used in mass sensitive biosensors. Where the elements are exposed to an atmosphere where desired molecules may be absorbed onto the element surface, this alters their weight and hence also their resonance frequency. It is therefore possible to get a very sensitive measure of weight.
Optical switch at the GHz level. In an array of a large number of elements positioned side by side with a spacing of the order of hundreds of nanometers, a grating is formed with spacing below the wavelength of light (of the order of 1000 nanometers). The spacing of the elements in such an arrangement determines an apparent refractive index of the structure as experienced by incident light. By applying appropriate voltages, the elements deform relative to one another in a non-linear way, which alters the apparent refractive index. This permits an optical switch to be fabricated allowing high frequency operation, of the order of GHz.